High performance aramid matrix composites

ABSTRACT

Thermoprocessible aramid matrix polymer based composites have high flex and short beam shear strengths and low moisture uptake.

RELATED APPLICATION

This application is a continuation-in-part of our pending applicationSer. No. 07/508,877, filed Apr. 12, 1990 U.S. Pat. No. 4,999,395.

BACKGROUND OF THE INVENTION

From a fabrication point of view, it is desirable to have highperformance composites prepared from thermoplastic matrix resin withfiber reinforcement such as p-aramid, carbon and glass. As compared tothermoset type matrix resins, such as epoxies and polyesters, thethermoplastics are thermoprocessible and in general, avoid the emissionof undesirable solvents. However, the thermoset resins offer certainqualities that are important to high performance composites such as highTg, high flex strength, high short beam shear and low moisture uptake.An object of the present invention is to provide a high performancecomposite from a thermoplastic resin whose properties are aboutequivalent to or exceed those of the thermoset matrix type composites.

SUMMARY OF THE INVENTION

This invention provides a high performance composite of a copolyamide ofthe following units: ##STR1## where n is 4 or 5; X is from 0.01 to 0.50,preferably from 0.03 to 0.30, and Ar is a radical selected from3,4'-oxydiphenylene, 4,4'-oxydiphenylene, 1,3-phenylene,1-methyl-2,4-phenylene, and mixtures of such radicals with each other orwith up to equimolar amounts of 1,4-phenylene radicals; reinforced withfrom about 10 to 90%, preferably 30 to 70%, by volume of fiber selectedfrom the group of glass, carbon and aramid fiber.

DETAILED DESCRIPTION OF THE INVENTION

The fiber reinforcement in composites of the invention can be continuousfilaments or staple fiber (cut lengths of varying or fixed lengths). Thecontinuous filament reinforcement used with this invention forstructural applications are glass, carbon or aramid filaments,preferably p-aramid filaments. The term "p-aramid" is used to designatearomatic polyamides whose chain extending bonds are either coaxial orparallel and oppositely directed. Useful p-aramids includepoly(p-phenylene terephthalamide) (PPD-T) as well as copolymers thereofwhich yield high strength, high modulus filaments. Other aramid fiberssuch as poly(m-phenylene isophthalamide) (MPD-I) fibers, especiallycrystalline MPD-I staple fibers (T-450 Nomex® aramid fibers having a cutlength of 3.8 cm and a linear density of 1.65 decitex, manufactured byE. I. du Pont de Nemours and Company) may also be employed. Glass fibersuch as a high strength E-glass, a lime-borosilicate glass derived froma Pyrex® composition, manufactured by Owens-Corning FiberglassCorporation can be used as yarn or woven fabric reinforcement.High-strength PAN-based carbon fiber (AS-4 produced by HerculesIncorporated) has been used as have other carbon fiber in yarn or inwoven form. The p-aramid filaments employed in Examples 1-3 arefinish-free, zero-twist, poly(p-phenylene terephthalamide) filaments(1420 denier, 1000 filament Kevlar® 49 aramid yarn manufactured by E. I.du Pont de Nemours and Company).

The composites may be prepared by any of a variety of techniques. Thus,the fiber may be combined with matrix polymer by solution coating; bymelt-extruding polymer over yarn; by cowinding filaments of matrixpolymer with reinforcing filaments into a sheet and melting the matrixpolymer filaments to form the matrix; by depositing the matrix polymerpowder on a warp of the reinforcing filaments and melting to providematrix; by extruding molten matrix polymer onto a warp of thereinforcing filaments; by applying films of matrix polymer to both sidesof a warp of the reinforcing filaments; etc. A plurality of suchreinforced yarns or sheets can be stacked or combined and formed underheat and pressure into a shaped reinforced structure. Similar techniquescan also be employed to combine the thermoplastic matrix with a fabricof glass, carbon or aramid filaments.

From about 30 to 70 volume percent of reinforcing filaments are normallyemployed in the composites to achieve maximum properties although lesseramounts will also exhibit the improvements contemplated herein.

The polymer matrix system employed in the present invention is acopolyamide having the following units: ##STR2## where n is 4 or 5; X isfrom 0.01 to 0.50, preferably from 0.03 to 0.30, and Ar is a radicalselected from 3,4'-oxydiphenylene, 1,3-phenylene,1-methyl-2,4-phenylene, and mixtures of such radicals with each other orwith up to equimolar amounts of 1,4-phenylene radicals. Its preparationis described in copending and coassigned U.S. application Ser. No.07/402,295. Preferred is the polymer in which Ar is 3,4'-oxydiphenyleneand the aliphatic unit is ##STR3## The polymer in which Ar is3,4'-oxydiphenylene, n is 5, and x is in the range of about 0.18 to 0.25is referred to below as Polymer A. It has an inherent viscosity of1-1.1. Small amounts of lactam are present as a result of the way thepolymer is prepared, however, for use in the examples below, the lactamwas extracted from the polymer before use. Also, the aliphatic contentof Polymer A was about 7.5 to 10.5 wt. %.

For composites of this invention appearing in Examples 1-3 below, thematrix polymer was first dissolved in dimethylacetamide (DMAc) to 4 wt.% polymer. The yarn bundle was then passed through this solution for apick-up of 10% by wt. of polymer with respect to yarn weight. The coatedfiber was dried to drive off the solvent and aligned to form pre-pregtapes of unidirectionally aligned fibers. Additional polymer may beapplied to build-up the resin content to the desired value in the finalcomposite laminate. One method of polymer build-up is to wrap the coatedfiber around a flat plate with a polymer film inserted betweensuccessive wrappings of the plate. The thickness of the film is dictatedby the desired matrix polymer volume loading. The wrapped plate issubjected to consolidation conditions of about 300° C., about 3800 kPa(550 psi) and a nitrogen purge to form two 15.2 cm by 15.2 cm (6 in. by6 in.) prepreg tapes. The pressure and heat was applied by a hydraulicplaten press. Other ways to build up resin volume are well known tothose skilled in the art.

Next, the composite is prepared by lay up of the prepreg plies in a moldto the desired fiber orientations, e.g., unidirectional, cross-ply, etc.and consolidation under heat and pressure as with a hydraulic platenpress. The conditions used were about 300° C. at about 1380 kPa (200psi) for about 15 minutes with an optional nitrogen purge. The resultingcomposite laminates were of good quality and appearance.

Test Procedures

The composite panels were cut into 15 cm×1.3 cm (6"×1/2") strips andsubjected to flexural modulus, flexural strength and short beam shear(SBS) testing as described in ASTM D2344 and D790 testing procedures.Water absorption was determined by weighting the samples at ambientconditions, placing the samples in 71° C. water for 13-14 days, thenreweighing the samples and calculating the percent water uptake. Theglass transition temperature (Tg) was determined by use of differentialscanning calorimetry (DSC) in accordance with ASTM D3418.

Definitions

In the examples, matrix Polymer A is as defined above. Polymer B ismetaphenylene isophthalamide (MPD-I) and Polymer C is a copolymer ofmetaphenylene diamine with a 70/30 mixture of isophthalic (I) andterephthalic (T) acids. Polymer D is an amorphous copolyamidecorresponding to Composition No. 3 in Table 1 of U.S. Pat. No.4,681,911. Polymer E is a commercially available epoxy resin such as3501-6 sold by Hercules, Inc.

The Tg of the various matrix polymers is reported in the followingtable.

    ______________________________________                                               Matrix Polymer                                                                          Tg                                                           ______________________________________                                               A         205                                                                 B         266                                                                 C         200                                                                 D         167                                                                 E         206                                                          ______________________________________                                    

The following examples, except for the controls are illustrative of thepresent invention and are not intended as limiting.

EXAMPLE 1

This example compares the flexural performance and short beam shear(interlaminar shear strength) performance of a number of matrix polymersreinforced with continuous paraphenylene terephthalamide (PPD-T)filaments (Kevlar® 49 aramid fiber). The fiber is aligned in aunidirectional manner to give the best values of strength. The resultsare presented in Table 1.

                  TABLE 1                                                         ______________________________________                                                  Fiber                                                                         Loading   Flex Strength SBS                                         Fiber Matrix                                                                            Vol. %    MPa (kpsi)    MPa (kpsi)                                  ______________________________________                                        A         61        724 (105)     61 (8.9)                                    B         67        593 (86)      38 (5.5)                                    C         67        627 (91)      35 (5.1)                                    D         60        696 (101)     59 (8.5)                                    E         60        655-689 (95-100)                                                                            59 (8.6)                                    ______________________________________                                    

As can be seen the Polymer A matrix system reinforced with PPD-T fibergives properties on a par with, to slightly better than the thermosetepoxy system E and thermoplastic matrix system D and is significantlybetter than the aramid matrix B based composite system.

EXAMPLE 2

This example compares the moisture uptake of PPD-T filament reinforcedPolymer A matrix composite with composites based on several otherpolymer matrices. The values are given in Table 2.

                  TABLE 2                                                         ______________________________________                                        Polymer Matrix                                                                              % Moisture Uptake                                               ______________________________________                                        A             1.3                                                             B             ≧4.0                                                     C             4.0                                                             D             ˜2.0                                                      E             2.0                                                             ______________________________________                                    

As can be seen the Polymer A matrix composite takes up the least amountof water. The moisture uptake conditions employed in testing the PolymerC matrix system was exposure to 95% relative humidity at 82° C. for 21days and conditions employed in testing the Polymer D matrix system wasexposure at 80% relative humidity at 82° C. for 21 days. It should benoted that these differ from those conditions used for the Polymer Amatrix system. The moisture uptake performances of Polymers B and E arewell known.

EXAMPLE 3

The importance of moisture uptake is evident in the retention ofmechanical properties. Table 3 compares the flexural strength of PolymerA matrix and Polymer D matrix reinforced with PPD-T fiber and calculatesthe percent retention of the strength after the conditioning mentionedin Example 2. The room temperature flex strength before conditioning istaken as the reference state and the flex strength at 93° C. is comparedto the reference state.

                  TABLE 3                                                         ______________________________________                                        Comparison of Elevated Temperature Flex Strength                              After Conditioning vs. Room Temperature Flex                                  Strength Prior to Conditioning                                                Flex Strength                                                                         R.T. Before   93° C. After                                                                      Strength                                             Conditioning  Conditioning                                                                             Retention                                    Matrix  MPa (kpsi)    MPa (kpsi) %                                            ______________________________________                                        A       662 (96)      483 (70)   74                                           D       696 (101)     441 (64)   63                                           ______________________________________                                    

As can be seen, the Polymer A matrix system retains significantly moreof its flex strength after conditioning and elevated temperature testingthan does the Polymer D matrix system.

EXAMPLE 4

A carbon fiber composite was made from Polymer A and AS-4 fiber. Noefforts were taken to remove the finish from the carbon fiber. Thevolume loading of the composite was ˜49% and the flexural andinterlaminar properties are listed in Table 4.

                  TABLE 4                                                         ______________________________________                                        AS-4 Carbon Fiber Reinforced Polymer A                                        Matrix Mechanical Properties                                                  ______________________________________                                        Flex Modulus   113    GPa       (16.4 Mpsi)                                   Flex Strength  1296   MPa       (188 kpsi)                                    SBS            72     MPa       (10.5 kpsi)                                   ______________________________________                                    

If the flex modulus and flex strength are normalized to 60% fiber v/othen the properties are comparable to other systems such as those basedon Polymer D or epoxy matrices.

                  TABLE 5                                                         ______________________________________                                        AS-4 Carbon Fiber Reinforced Polymers                                                             Polymer A                                                 Property  Exp.      Normalized                                                                              Polymer D                                                                             Epoxy                                   ______________________________________                                        Fiber v/o, %                                                                            49        @ 60      ˜60                                                                             ˜60                               Flex Modulus,                                                                           .sup. 113 (16.4)                                                                        138 (20)  103-138 103-138                                 GPa (Mpsi)                    (15-20) (15-20)                                 Flex Strength                                                                           1296 (188)                                                                              1586 (230)                                                                              1310-1448                                                                             1517                                    MPa (kpsi)                    (190-210)                                                                             (220)                                   ______________________________________                                    

EXAMPLE 5

A sample of Polymer A, vacuum extracted so that it contained less than6% residual free caprolactam, was fed through a single screw meltextruder and a yarn of continuous MPD-I filaments having an individualfilament linear density of 1.65 decitex and a yarn linear density of 132tex was fed through a die to coat the filaments. The yarn was dried inan oven at 120° C. for 48 hours to lower the moisture content to lessthan 2%. The yarn linear speed, die size, and extruder screw speed werevaried to obtain the desired melt coating thickness and quality. Theyarn linear speed was varied from 6.1 m/min (20 ft./min) to 30.5 m/min(100 ft./min.). It was concluded that 15.2 m/min (50 ft./min.) was theoptimum linear speed for coating uniformity and filament bundlecoverage. The initial yarn thickness was about 0.41 mm (16 mils) andenough coating was applied in the melt extrusion process to increase thebundle thickness to 0.91 mm (36 mils). The Polymer A matrix resin wasextruded with an extruder temperature profile of 277° C. (530° F.) to291° C. (555° F.), a die temperature of 285° C. (545° F.), and a polymermelt temperature of 308° C. (587° F.) at the point of extrusion onto thefilaments. The fact that the temperature at the point of extrusion washigher than the die melt temperature indicated that there was heatgeneration in the die owing to significant melt viscosity of the matrixresin.

The procedure was repeated, except that a yarn of continuous PPD-Tfilaments having an individual filament linear density of 1.65 decitexand a yarn linear density of 330 tex was used in place of the yarn ofcontinuous MPD-I filaments. The initial yarn thickness was about 0.91 mm(36 mils) and enough coating was applied in the melt extrusion processto increase the bundle thickness to 1.32 mm (52 mils). The Polymer Amatrix resin was extruded with an extruder temperature profile of 249°C. (480° F.) to 266° C. (510° F.), a die temperature of 260° C. (500°F.), and a polymer melt temperature of 282° C. (540° F.) at the point ofextrusion onto the filaments. The linear speed of the filament bundlewas 6.1 m/min (20 ft./min).

The coated filament bundles from the above procedures could be cut intoflakes having a size of about 0.3 cm to 1.3 cm for extrusion orcompression molding.

EXAMPLE 6

A matrix polymer according to formula I in which Ar was3,4'-oxydiphenylene, n was 4, x was about 0.093, and the inherentviscosity was about 1.1 was used to make a unidirectional compositereinforced with PPD-T continuous filaments. The volume loading of theresulting composite was nominally 60%, and the glass transitiontemperature was 230° C.

EXAMPLE 7

A thin composite was made by sandwiching a piece of graphite fabricbetween two sheets of film of Polymer A. This combination was subjectedto approximately 200 psi pressure at 300° C. for 15 minutes. Theresulting laminate was approximately 0.4 mm (15.6 mils) thick and hadgood appearance. The graphite fiber was of the AS-4 variety and theweave was a 4-harness satin (crows foot).

EXAMPLE 8

A unidirectional composite was prepared from E-glass as reinforcement ina matrix resin similar to that used in Example 1. The fiber was coatedwith matrix resin by dipping a glass fiber bundle in a dilute solution.The coated fiber was wound on a card and more solution applied to buildup the resin loading. The resulting card wound tapes were laid in aclosed mold and consolidated under heat and pressure to form a laminatecomposed of unidirectional plies. The fiber column was about 62%. Thecomposite exhibited the following properties:

Flex modulus--47.6 GPa (6.9 Mpsi)

Flex strength--1470 MPa (212.8 kpsi)

SBS--87.6 MPa (12.7 kpsi)

EXAMPLE 9

In this example a matrix polymer according to Formula I was made inwhich n was 5, Ar was a mixture of 70% m-phenylene and 30% p-phenylene,x was 0.27, and the inherent viscosity was 0.8. This matrix polymer wasused to make a unidirectional composite reinforced with graphitecontinuous filaments. The composite so made had excellent short beamshear and flexural performance.

Into a two-liter resin kettle fitted with a stirrer and heating mantlewas placed a mixture of N,N'-isophthaloyl bis-caprolactam (862.5 g, 2.4mol), m-phenylenediamine (183.2 g, 1.7 mol), and p-phenylenediamine(78.5 g, 0.73 mol). The mixture was maintained under a continuousnitrogen flow at a temperature between 250° and 260° C. for four hours.The product was a clear amber plasticized copolymer in solution with34.3% caprolactam by weight. The inherent viscosity of the copolymer wasdetermined to be 0.8, its Tg was 217° C., and the value of x in theformula of the composition was determined from its proton-NMR spectrumusing the method described in U.S. patent application Ser. No.07/402,295, filed 9/5/89.

A solution of the above copolymer was prepared by combining about 200 gof the plasticized copolymer with about 1500 g of DMAc and shaking themtogether at room temperature until a clear, light brown/gold solutionwas obtained. In each of a series of batch runs, a Waring 7011 blenderwas filled with about 250 mL of distilled water. With the blender run onhigh speed, about 50 mL of copolymer solution was poured slowly into theblender into the agitated aqueous solution. The product formed was apowder, which was filtered and washed with about 500 mL of water anddried in a 110° C. vacuum oven.

A total of 140 g of the copolymer powder made in this manner wascombined to be used as the matrix polymer to make the unidirectionalcomposite. The density of the copolymer was 1.2965 g/mL, and it wasfound to contain 0.18 wt. % free caprolactam. A solution containing 4wt. % of the copolymer in DMAc was prepared. High strength PAN-basedcarbon fiber yarn (AS-4 yarn, Hercules Incorporated) was passed thoughthe solution of the copolymer for a pick-up of 7% by wt. of copolymerwith respect to yarn weight. The coated yarn was dried to drive off thesolvent and aligned to form twelve 15.2 cm×15.2 cm (6 in.×6 in.) prepregtapes. The loading of the copolymer on the yarn was increased byadditional coating so that the final fiber/copolymer content was 60/40on a weight basis. A unidirectional composite was then made from thetwelve prepreg plies at a maximum temperature of 300° C. and a maximumpressure of 83,000 kPa (12,000 psi).

The composite exhibited the following properties:

Flex modulus--131 GPa (19 Mpsi)

Flex strength--1860 GPa (270 Kpsi)

SBS--103-124 (15-18 Kpsi).

We claim:
 1. An aramid, carbon or glass fiber coated with a polymerconsisting essentially of a copolyamide of the following units: ##STR4##where n is 4 or 5; X is from 0.01 to 0.50, preferably from 0.03 to 0.30,and Ar is a radical selected from 3,4'-oxydiphenylene,4,4'-oxydiphenylene, 1,3-phenylene, 1-methyl-2,4-phenylene, and mixturesof such radicals with each other or with up to equimolar amounts of1,4-phenylene radicals.
 2. An aramid fiber according to claim 1 in whichAr is 3,4'-oxydiphenylene and the aliphatic unit is ##STR5##